HANDS:
hands
hands title:
§10. Invisible war
hands date:
10 May 2023
hands text:
`If a cat can kill a rat in a minute, how long would it be killing 60,000 rats?'
Ah, how long, indeed! My private opinion is that the rats would kill the cat.
Ah, how long, indeed! My private opinion is that the rats would kill the cat.
01
We live in a strange world. At times it seems as if there is an invisible war within the construction industry.
02
Although manufacturers are very clear on whether or not you can tile on gypsum based substrate, the debate is nowhere near the end.
03
Since gypsum is the most popular plaster in the UK, the problem is widespread. It is, probably, the most common cause of tile failure. The following, of course, concerns any calcium sulfate based surfaces, that is anhydrit, hemihydrate, alpha-hemihydrate, gyvlon screeds, gypsum based bonding coats, plasters and so on.
04
There is a good reason why tiling on gypsum is not a good idea.
05
Gypsum contains calcium sulphate that reacts with calcium aluminate in the cement based adhesive - most tile adhesives sold in the UK are cement based - to form ettringite or calcium sulfoaluminate, little white of yellow crystals. Excessive amount of ettringite can cause the adhesive to fail.
06
It is well known that gypsum is, in fact, added into the cement. The idea is to prevent cement from setting too quickly and, therefore, gypsum acts as retarding agent.
07
However, the amount of gypsum in Portland cement is very small and never goes above 4-5%. When the level of gypsum is increased, the concrete tends to fail.
08
If, nevertheless, you have no choice but tile on a gypsum based substrate, the following options can be considered: 1) use gypsum based adhesive; and/or 2) create a barrier between the substrate and the adhesive. Such barrier can be created with a special polymer based primer or a membrane.